Various methods of spinal immobilization have been used during this century in the treatment of spinal instability and displacement. The most common treatment for spinal stabilization is immobilization of the joint by surgical fusion, or arthrodesis. This has been known for almost a century. In many cases, however, pseudoarthrosis is a problem, particularly in cases involving fusion across the lumbosacral articulation and when more than two vertebrae are fused together. Early in the century, post operative external immobilization such as the use of splints and casts was the favored method of spinal fixation. As surgical techniques became more sophisticated, various methods of internal and external fixation were developed.
Internal fixation refers to therapeutic methods of stabilization that are wholly internal to the patient and include commonly known devices such as bone plates, screws, rods and pins. External fixation, in contrast, involves at least some portion of the stabilization device being located external to the patient's body. As surgical technologies and procedures became more advanced and the likelihood of infection decreased, internal fixation became the favored method of immobilization since it is less restrictive on the patient.
Internal fixation of the spine may be used to treat a variety of disorders including kyphosis, spondylolisthesis and rotation, segmental instability, such as disc degeneration and/or fracture caused by disease, trauma, congenital defects and tumor diseases.
One of the main challenges associated with spinal fixation is securing the fixation device to the spine without damaging the spinal cord. The pedicles of a vertebra are commonly used for fixation as they generally offer an area that is strong enough to hold the fixation device in place to fix the treatment area even when the patient suffers from degenerative instability such as osteoporosis. Early fixation devices involved the use of screws extending through the facets and into the pedicles.
Current fixation devices and hardware systems used internally for spinal fixation in modern surgical procedures are generally designed to meet one or more criteria, such as: providing rigidity as is indicated, generally along the long axis of the patient's spine; accommodating a broad variation in the size and shape of the spinal member with which it is used; having the capability of handling the stresses and strains to which the devices will be subjected resulting from movement of the spine; and providing easy surgical access during both implantation and removal of the implant.
Of these factors, the most difficult to achieve may very well be providing easy surgical access when the implant is being deployed and/or removed by surgeon. In particular, surgeons often wish to fit, test, adjust and refit fixation devices numerous times during a procedure in order to ensure that the device is optimally positioned. This is particularly important when dealing with the spinal column due to the risk of paralysis.
One example of a device designed as an attempt to meet the above-described criteria is disclosed in U.S. Pat. No. 5,466,237. The fixation device disclosed includes what is described as a variable position locking anchor having a bone screw and a seat.
The disclosed device uses a nut as a locking mechanism. As disclosed, the nut is tightened to the seat to compress an attached rod along a longitudinal axis of the screw, causing the screw to engage in mating contact with the seat and thereby locking the screw in place. In use, it is difficult to maintain proper positioning of the fixation device while tightening a nut or other such locking mechanism. Moreover, many existing fixation devices suffer from splaying apart of the sides of the seat of the fixation device when compressive force is used to locking a rod in position with a screw. The sides of the seat can splay apart and creates gaps in the fixation device and decreases its effectiveness. It would preferable if the fixation device was resistant to splaying.
It would be preferable if the screw could be locked without having to generate the necessary compressive force by tightening the nut or other such locking mechanism. It would also be preferable if the locking mechanism included a partial or preliminary lock for assisting the surgeon in fitting the implant prior to finally locking the implant in place. It would further be preferable if the locking mechanism was designed to prevent, or at the very least, minimize splaying of the various parts of the locking mechanism.
The present invention includes a novel fixation device that overcomes the disadvantages of the prior art.